Various disklike read-only or rewritable storage media, including optical disks such as a CDs and DVDs to read and write data using a light beam, magnetic disks such as flexible disks to read and write data using magnetism, and magneto-optical disks such as MOs and MDs to read and write data using a light beam and magnetism in combination, have already been used extensively around the world. Among other things, DVD-RAMs, MOs and other recordable media are housed in a cartridge such as the one disclosed in Patent Document No. 1 in order to protect the data stored there. As used herein, a “disk cartridge” refers to a cartridge in which such a disklike read-only or rewritable storage medium is housed.
FIG. 24 schematically illustrates the structure of the disk cartridge that is disclosed in Patent Document No. 1. As shown in FIG. 24, the disk cartridge 100 includes a supporting base member 101 that houses a read-only or rewritable disk 10 and that forms the outer shell of the disk cartridge 100.
The supporting base member 101 has a window 101w on the upper and lower surfaces thereof to allow some means for rotating the disk 10 (such as a disk motor) and a read/write means to enter the supporting base member 101 and access the disk 10.
The supporting base member 101 also has inner walls for holding the disk 10 rotatably with the center of the disk 10 positioned and with some clearance left with respect to the disk 10.
The disk cartridge 100 includes a shutter 103 that has been folded in a C-shape to close the window 101w on the upper and lower surfaces of the supporting base member 101 and to protect the data side of the disk 10 to be exposed inside the window 101w. 
The shutter 103 can move parallel in the direction indicated by the arrow P in FIG. 24 and is biased by a spring so as to keep the window 101w closed unless some external force is applied thereto. Thus, this disk cartridge 100 is designed so as not to expose the disk 10 unless the external force is applied.
To perform a read/write operation on this disk cartridge 100 using a disk drive (not shown), the shutter 103 of the cartridge is moved in the direction indicated by the arrow P and the disk 10 is exposed in order to clamp the disk 10 and to allow a read/write means to access the disk 10. In this case, as the disk cartridge 100 being inserted into the disk drive goes in the direction indicated by the arrow Q, the notched portion 103a of the shutter 103 gets engaged with the protrusion 104a of a shutter opener 104 and the shutter opener 104 is turned around the center of rotation 104b in the direction indicated by the arrow R, thereby sliding the shutter in the direction indicated by the arrow P.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 9-153264